1. Field of the Invention
This invention relates to a process for selectively separating gaseous components from a gaseous mixture by using an adsorbent.
2. Description of the Prior Art
As processes for the selective separation of gaseous components, especially gaseous impurities from a gaseous mixture, it has heretofore been the common practice to have the gaseous impurity adsorbed on an adsorbent in an adsorbent layer and, when the adsorbent has reached the limit of its adsorbing ability, to depressurize the adsorbent layer, followed by purging of the adsorbent layer with a gas which does not contain the gaseous impurity so much, so that the gaseous impurity is desorbed from the adsorbent to regenerate the adsorbent.
Among these processes, Japanese Patent Publication No. 47051/1987 (hereinafter abbreviated '051) discloses the following process as a gas separation process capable of minimizing the loss of the treated, i.e., purified gas.
According to this conventional process, the gaseous impurity is selectively and adiabatically adsorbed on an adsorbent. The adsorbent is depressurized and is then purged at a low pressure using a gas which may range from a gas contaminated only a little to a substantially pure gas, thereby conducting desorption of the gaseous impurity and regeneration of the adsorbent. The adsorbent is pressurized again. The conventional process features alternate and cyclic use of many columns with adsorbents so that the gaseous impurity is separated from a gaseous mixture to provide a purified gas. Each cycle comprises a number of steps starting with a first adsorption column whose adsorbent has fatigued as a result of adsorption of the gaseous impurity. These steps comprise:
depressurizing the first adsorption column through an outlet thereof while an inlet of the first adsorption column is closed, so that gas present in the voids of the first adsorption column is allowed to expand; PA1 introducing the thus-expanded gas to another adsorption column, which has been regenerated, through an outlet of said another adsorption column until said another adsorption column and the first adsorption column are equalized in pressure; PA1 depressurizing the first adsorption column further through the outlet thereof to cause the gas, which is present in the voids of the first adsorption column, to expand further and, then, introducing the further-expanded gas into a packed column with an inert, non-porous packing having a high void fraction from one end thereof; PA1 introducing another further-expanded gas from a second adsorption column, whose adsorbent has fatigued, to the packed column through the opposite end of the packed column, whereby eliminating the first-mentioned further-expanded gas from the packed column; PA1 depressurizing the first adsorption column to a low pressure through the inlet thereof, introducing a portion or all of the first-mentioned further-expanded gas, which has been eliminated from the packed column, into the first adsorption column to purge the first adsorption column, and if there is still any remaining portion of the first-mentioned further-expanded gas eliminated from the packed column, introducing the remaining portion of the first-mentioned further-expanded gas into a third adsorption column, which has been regenerated, through an inlet thereof to pressurize the third adsorption column to an intermediate level; PA1 introducing expanded gas from a fourth adsorption column, which has fatigued, into the first adsorption column, which has already been regenerated, through the outlet while closing the inlet of the first adsorption column until the fourth adsorption column and the first adsorption column are equalized in pressure; PA1 introducing a gas of the same quality as the purified gas from a gas stream of the same quality as the purified gas into the first adsorption column through the outlet thereof while closing the inlet of the first adsorption column until the pressure inside the first adsorption column becomes equal to the pressure of the gas stream; and PA1 introducing the gaseous mixture, which contains the gaseous impurity, into the first adsorption column through the inlet thereof and discharging the purified gas through the outlet of the first adsorption column. PA1 a) feeding the gaseous mixture into a first adsorption column through an inlet thereof, discharging the purified gas through an outlet of the first adsorption column, and terminating the feeding and discharging before the first adsorption column has no remaining adsorbing ability and the purity of the purified gas falls below a target level; PA1 b) communicating an outlet of at least one other adsorption column, whose adsorbent has been regenerated, with the outlet of the first adsorption column already subjected to the step a, whereby both the adsorption columns are equalized in pressure, and then eliminating the communication; PA1 c) communicating the outlet of the first adsorption column, which has been subjected to step b, with a first end of the holding column which contains a gas received as a result of communication of a second end of the holding column with at least one outlet of the other adsorption columns having been subjected to steps a and b like the first adsorption column, whereby the gas inside the first adsorption column is allowed to enter the holding column; during the entrance of the gas from the first adsorption column into the holding column, maintaining the second end of the holding column in communication with an outlet of a second adsorption column already subjected to steps a, b and c and also to the below-described step d, whereby the second adsorption column is purged with the gas forced out of the holding column by the gas flowed into the holding column, the last-mentioned gas as needed; and, until the breakthrough of the components adsorbed on the adsorbent bed of the first adsorption column begins to occur in the gas, the gas in the first adsorption column being allowed to enter the second adsorption column through the outlet thereof by the holding column or without allowing a portion of the gas of the first adsorption column to flow by the holding column, thereby purging the second adsorption column; PA1 d) closing a purge valve of the second adsorption column as needed, whereby the second adsorption column and the first adsorption column are communicated to each other and equalized in pressure, and then eliminating the communication; PA1 e) closing the outlet of the first adsorption column already subjected to step c or d and depressurizing the first adsorption column to a low pressure by a purge valve provided on the side of the inlet of the first adsorption column, whereby the components adsorbed on the adsorbent bed of the first adsorption column is desorbed and eliminated; PA1 f) communicating an outlet of a third adsorption column, which has been subjected to steps a and b like the first adsorption column, with the first end of the holding column, whereby the gas inside the third adsorption column is allowed to enter the holding column until the breakthrough of the components adsorbed on the adsorbent bed of the third adsorption column begins to occur in the gas, said gas entering the holding column optionally containing a portion of the purified gas as needed; during the entrance of the gas from the first adsorption column into the holding column, maintaining the second end of the holding column in communication with the outlet of the first adsorption column already subjected to step e, whereby the first adsorption column is purged with the gas forced out of the holding column by the gas flowed into the holding column, the last-mentioned gas optionally containing a portion of the purified gas as needed; and, until the breakthrough of the components adsorbed on the adsorbent bed of the third adsorption column begins to occur in the gas, the gas in the third adsorption column being allowed to enter the first adsorption column by the holding column or without allowing a portion of the third adsorption column gas to flow by the holding column, thereby purging the first adsorption column; PA1 g) closing the purge valve of the first adsorption column as needed, whereby the first adsorption column and the third adsorption column are communicated to communicate with each other and equalize in pressure, and then eliminating the communication; PA1 h) communicating the outlet of at least one other adsorption column, which has been subjected to step a or steps a and b like the first adsorption column, to the outlet of the first adsorption column, which has been subjected to step f or g, while closing both an inlet of said at least one other adsorption column and the inlet of the first adsorption column, whereby the gas inside said at least one other adsorption column is allowed to enter the first adsorption column, said last-mentioned gas optionally containing a portion of the purified gas as needed, to subject both said at least one other adsorption column and the first adsorption column to at least one pressure equalizing operation, and then eliminating the communication; and PA1 i) causing a gas of the same quality as the purified gas to flow into the first adsorption column, which has been subjected to step h, through the outlet thereof while closing the inlet thereof, whereby the pressure of the first adsorption column is equalized with that of the gas of the same quality as the purified gas, and then closing the outlet of the first adsorption column.
According to the above conventional process, the use of the column packed with the inert, non-porous packing having the high void fraction as the holding column of the feed-in/feed-out sequence retaining type has made it possible to reverse the concentration gradient opposite to that at the time of the flowing-in, thereby having the merit that the loss of the purified gas can be reduced.
The '051, however, does not contain any specific disclosure about the size of the packed column. In claim 2 of this citation, there is a disclosure that "the expanded gas introduced from the first adsorption column through one inlet of the packed column, which can be approached in two directions, substitutes for and forces out the gas introduced from another adsorption column through the another inlet of the packed column in the preceding step, and the gas so substituted and forced out is entered another adsorption column to effect purging and/or repressurization." As is understood from the disclosure, the packed column satisfactorily performs the above function in the conventional process so that its capacity is considered to be large enough to hold the whole amount of the purge gas sufficiently.
It is, however, not advantageous from the economical standpoint to provide a packed column having a capacity sufficient to hold the whole amount of purge gas in view of safety. Further, from the technical standpoint, it is not clear if the capacity sufficient to hold the whole amount of purge gas is needed.